1. Field of the Invention
The present invention relates to an object, a method, a method for producing an object, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. More particularly, the present invention relates to, for example, a semiconductor material and a semiconductor device using the semiconductor material.
In this specification, a “semiconductor device” generally refers to a device which can function by utilizing semiconductor characteristics; a transistor, a diode, a photoelectric conversion device, an electro-optical device, a light-emitting display device, a memory device, an imaging device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Attention has been focused on a technique for forming a photoelectric conversion device and a transistor using semiconductor thin films. The transistor is applied to a wide range of semiconductor electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a photoelectric conversion device and a transistor. As another material, an oxide semiconductor has attracted attention.
For example, a technique in which a transistor is manufactured using a Zn—O-based oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).
It is known that many oxide semiconductors have n-type conductivity. Examples of oxide semiconductors having n-type conductivity include materials such as ZnO, In2O3, SnO2, GaO, TeO, GeO2, WO3, and MoO3.
On the other hand, as oxide semiconductors having p-type conductivity, materials such as ZnO, CuAlO2, NiO, and IrO are known.
Further, a photoelectric conversion device that generates power without carbon dioxide emissions and that does not generate any harmful emissions has attracted attention as a countermeasure against global warming. As a typical example of the photoelectric conversion device, a silicon (Si) solar cell which uses single crystal silicon, polycrystalline silicon, or the like has been known, and has been actively researched and developed.
In a solar cell using a silicon substrate, a structure having a p-n homojunction is widely used. Such a structure is formed by diffusion of impurities having a conductivity type opposite to that of the silicon substrate into one surface of the silicon substrate.
On the other hand, in order to improve output voltage in power generation, a structure of a solar cell having a p-n heterojunction in which a wide-gap semiconductor material provided as a window layer and a silicon substrate of a photoelectric conversion layer are combined is known (see Non-Patent Document 1). The p-n heterojunction is formed by formation of a wide-gap semiconductor having a different band gap and conductivity type from those of the silicon substrate on one surface of the silicon substrate.
Further, a structure of a silicon solar cell having a semiconductor-insulator-semiconductor (SIS) structure in which a p-n heterojunction is formed with a thin insulating film provided therebetween is known (see Non-Patent Document 2).